Aerodynamically stabilized tow assembly for aircraft for geophysical surveying

ABSTRACT

A airborne geophysical electromagnetic (EM) survey tow assembly system for use with an aircraft, including a substantially rigid receiver coil frame that defines a central open area, the receiver coil frame forming a continuous internal passageway that extends around the central open area; a receiver coil housed within the internal passageway isolated from the central open area, the receiver coil being configured to measure the response of surveyed terrain to naturally occurring EM events; and a tow cable for suspending the receiver coil frame from the aircraft, the receiver coil frame being formed from rigid members configured to provide aerodynamic stabilization to maintain the receiver coil frame in a desired pitch and yaw orientation relative to a direction of travel and the horizontal when the receiver coil frame is suspended during flight.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/910,386 filed Oct. 22, 2010, issued as U.S. Pat. No. 8,493,068 onJul. 23, 2013, and claims the benefit of and priority to U.S.Provisional Patent Application No. 61/254,451 filed Oct. 23, 2009, underthe title TOW ASSEMBLY FOR FIXED WING AIRCRAFT FOR GEOPHYSICALSURVEYING; the contents of each of the above patent applications arehereby expressly incorporated by reference into the detailed descriptionhereof.

FIELD

This description relates to a receiver coil tow assembly system for usewith an aircraft for geophysical surveying.

BACKGROUND OF THE INVENTION

Geophysical electromagnetic (“EM”) prospecting techniques can beeffective in determining the electrical conductivity of soils, rocks,and other bodies at and under the earth's surface.

Geophysical EM prospecting can be carried out using surface basedequipment and airborne equipment. Airborne methods in which equipment istransported by aircraft such as helicopter, airplane or airship may beuseful for large area surveys. For airborne electromagnetic (“AEM”)systems, survey data may be acquired while an airplane or helicopterflies at a nearly constant speed along nearly-parallel and close toequally-spaced lines at an approximately constant height above ground.

Some geophysical surveying methods are active in that the equipment isused to transmit a signal to a targeted area, and then measure aresponse to the transmitted signal. Other geophysical surveying methodsare passive in that signals produced from a target area are measuredwithout first transmitting a signal to the target area. An example of apassive geophysical EM prospecting method is Audio Frequency Magnetic(“AFMAG”) surveying in which the EM fields resulting from naturallyoccurring primary signal sources such as lightning discharges aremeasured. These EM fields propagate around the earth as plane wavesguided by the ionosphere and earth's surface. Lightning activityoccurring remote from the measurement point can produce signals with anearly flat spectral density at frequencies between, for example, 8 Hzand 500 Hz, varying with geographical location, time of the day, seasonsand weather conditions. An example of a passive AFMAG geophysical EMprospecting method is shown in U.S. Pat. No. 6,876,202.

A tow assembly that can be efficiently used in conjunction with a fixedwing aircraft is desirable. A tow assembly that is aerodynamicallystable when suspended from an aircraft is also desirable.

SUMMARY

According to one example embodiment is an airborne geophysicalelectromagnetic (EM) survey tow assembly system for use with anaircraft. The system comprises a substantially rigid receiver coil framethat defines a central open area, the receiver coil frame forming acontinuous internal passageway that extends around the central openarea; a receiver coil housed within the internal passageway isolatedfrom the central open area, the receiver coil being configured tomeasure the response of surveyed terrain to naturally occurring EMevents; and a tow cable for suspending the receiver coil frame from theaircraft, the receiver coil frame being formed from rigid membersconfigured to provide aerodynamic stabilization to maintain the receivercoil frame in a desired pitch and yaw orientation relative to adirection of travel and the horizontal when the receiver coil frame issuspended during flight.

According to another example embodiment is an airborne geophysicalelectromagnetic (EM) survey system, comprising: an aircraft; asubstantially rigid receiver coil frame that defines a central openarea, the receiver coil frame forming a continuous internal passagewaythat extends around the central open area; a receiver coil housed withinthe internal passageway isolated from the central open area; a tow cablefor suspending the receiver coil frame from the aircraft; and signalprocessing equipment in communication with the receiver coil to receivesignals therefrom representative of EM fields generated by a surveyedterrain in response to naturally occurring electrical events; thereceiver coil frame being formed from rigid members configured toprovide aerodynamic stabilization to maintain the receiver coil frame ina desired pitch and yaw orientation relative to a direction of traveland the horizontal when the receiver coil frame is suspended duringflight.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments are provided in the following description. Suchdescription makes reference to the annexed drawings wherein:

FIG. 1 is a perspective view of an airborne geophysical electromagnetic(EM) survey tow assembly system, including a receiver coil assemblysuspended from a tow and latch assembly that is secured to a fixed wingaircraft, according to example embodiments.

FIG. 2 is a perspective view of the airborne geophysical survey towassembly system of FIG. 1, with the receiver coil assembly nested in thetow and latch assembly that is secured to the fixed wing aircraft.

FIG. 3 is a perspective view of the airborne geophysical survey towassembly system of FIG. 1, with the receiver coil assembly nested in thetow and latch assembly.

FIG. 3A is a perspective view of a latch system of the tow and latchassembly of the airborne geophysical survey tow assembly system of FIG.1.

FIG. 4 is a perspective view of the tow assembly of the airbornegeophysical survey receiver coil assembly system of FIG. 1.

FIG. 5 shows a representation of an AFMAG geophysical prospecting systemthat incorporates the airborne geophysical survey tow assembly system ofFIG. 1, according to one example embodiment of the invention.

FIG. 6 is a sectional view of one of the side members of the receivercoil assembly taken along the lines VI-VI of FIG. 4, according to anexample embodiment.

FIG. 7 is a sectional view of one of the side members of the receivercoil assembly taken along lines VII-VII of FIG. 6.

FIG. 8 is a sectional view of one of the side members of the receivercoil assembly taken along the lines VI-VI of FIG. 4, according toanother example embodiment.

FIG. 9 is a sectional view of one of the side members of the receivercoil assembly taken along lines X-X of FIG. 8.

FIG. 10 is a sectional view of one of the side members of the receivercoil assembly according to another example embodiment.

DESCRIPTION OF EXAMPLE EMBODIMENTS

FIGS. 1 and 2 show an airborne geophysical electromagnetic (EM) surveytow assembly system 100 that includes a receiver coil assembly 112 and atow and latch assembly 102, according to example embodiments of theinvention. The tow and latch assembly 102 is secured to the underside ofa fixed wing aircraft 104 and includes a winch system 105 with aretractable tow cable 110 for suspending and retracting the receivercoil assembly 112. The winch system 105 can for example include ahydraulically driven winch that is located inside the body of theaircraft, with the tow cable 110 extending through an opening 106 in theaircraft body. The receiver coil assembly 112 is shown hanging from towcable 110 from the tow and latch assembly 102 in a suspended position inFIG. 1. In particular, in FIG. 1 the receiver coil assembly 112 is shownwith the tow cable 110 being in a partially extended or partiallyretracted position as the receiver coil assembly 112 is being deployedfrom or retracted back to the aircraft. The receiver coil assembly 112may by way of non limiting example be suspended about 100 meters (orgreater or less amounts) for flying a geophysical survey. FIG. 2 showsthe receiver coil assembly 112 in a retracted position secured by towand latch assembly 102 to an underside of the aircraft 104. Typicallythe receiver coil assembly 112 will be kept in the retracted positionwhen a survey is not being conducted—for example when flying to or froma survey location and during take-off and landing. Suspending thereceiver coil assembly 112 as shown in FIG. 1 during a geophysicalsurvey mitigates against noise caused by the towing aircraft 104.

Referring to FIGS. 1 and 4, the receiver coil assembly 112 includes asubstantially rigid aerodynamic tubular receiver coil frame 120 thatdefines a rectangular internal passageway 124 in which a rectangularmulti-turn receiver loop or coil 116 (illustrated by dashed lines) ishoused. As will be appreciated from the Figures, the receiver coil frame120 forms a loop such that internal passageway 124 is a continuousclosed loop. In the illustrated embodiment, the receiver coil frame 120has a substantially rectangular shape that defines the perimeter of acentral open area or opening 122. The internal passageway 124 extendsaround the central open area 122 and air can pass through the centralopen area 122. The rectangular receiver coil frame 120 is formed by apair of parallel tubular side frame members 130, 132 interconnected byfront and back parallel tubular frame members 134, 136. In theillustrated embodiment, the tubular side frame members 130, 132 arelonger than the front and back parallel tubular frame members 134, 136,and vertical stabilizing fins 140 are positioned near the back ortrailing end of the rectangular receiver coil frame 120 to assist inkeeping the frame oriented in a consistent direction during flight withthe receiver coil 120 in a nominally horizontal orientation with itsaxis vertically oriented.

In an example embodiment, a central boom in the form of an elongatesupport member 142 extends from front tubular frame member 134 to backtubular frame member 136 across the middle of the central opening 122.As illustrated embodiment, the central support member 142 may be locatedsubstantially between and parallel to the side tubular frame members130, 132, and includes a central tow cable fastener 144 to which the towcable 110 is connected. Each of the tubular frame members 130, 132, 134and 136 and the support member 142 may be shaped to provide the receivercoil tow assembly 112 with a shape that mitigates air-flow resistanceand vibration and also maintains the tow assembly in a consistentorientation while it is being towed. In this regard, the tubular memberscan be streamlined and faired relative to their respective orientationwithin the receiver coil frame—for example back tubular frame member 136may have a flat wing-like configuration. The tubular frame members 130,132, 134 and 136 and the support member 142 can be formed fromsubstantially from materials that are electrically insulating andnon-magnetic such as fiberglass.

Referring to FIGS. 1-3A, the tow and latch assembly 102 includes amechanically or hydraulically driven latch system 150. As best shown inFIG. 3A, in the illustrated embodiment, the latch system 150 includesspaced apart front and back latch members 152, 154 for securely engagingthe central support member 142 of the receiver coil assembly 112 whenthe receiver coil assembly is in its retracted, latched position underthe aircraft 104. Each of the front and back latch members 152, 154include a respective pair of opposed, pivotally mounted latch arms 158for engaging opposite sides of the outer surface of the central supportmember 142, and an upper cradle member 156 for engaging a top surface ofthe central support member 142. The latch arms 158 can include diverginglower ends to act as a guide for central support member 142 as thereceiver coil assembly is moved into or out of its retracted position.In the illustrated embodiment, tie rods 162 may be used to mechanicallylink the latch arms 158 so that the front and back latch members 152,154 operate in unison. The latch system 150 includes a strut system 160for securing it to the aircraft 104. Alternative latch systemconfigurations can be used to secure the receiver coil assembly 112 tothe aircraft, the above described latch system being but one example.

Turning again to the receiver coil frame 120, in an example embodimentthe frame is configured so that it can be split along a horizontal planeto allow the receiver coil 136 to be inserted, serviced and removed fromthe rectangular internal coil passageway 124. In at least some exampleembodiments elastic suspension system is used to secure the receivercoil 116 within the rectangular internal coil passageway 124, and inthis regard FIGS. 6-10 illustrate three different possible receiver coilsuspension systems.

A single suspension receiver coil suspension system is illustrated inFIGS. 6 and 7. Such system includes a rectangular inner frame member 12Aelastically suspended at the center of the coil passage 124 by multiplefastening assemblies 40 that are spaced internally along the length ofeach of the tubular frame members 130, 132, 134 and 136 (although shownas having a circular cross-section in FIG. 6, such members can havedifferent cross-sectional shapes as will be appreciated from theprevious Figures). Rectangular inner frame member 12A may be a rigidopen-topped trough in which the receiver coil is secured. Each fasteningassembly 40 includes an elastic suspension member 32 that extendsbetween the inner wall of the tubular frame member 130, 132, 134 and 136and the inner frame member 12A. In one example embodiment, each elasticsuspension member 32 (which may for example be formed from rubber) issecured at opposite first and second ends 38 to longitudinally spacedlocations on a side of the inner frame member 12A, and at an approximatemid-point 36 to the inner wall of the tubular frame member 130, 132, 134and 136 such that the elastic suspension member 32 forms a “V” shapethat applies opposing longitudinal forces to the inner frame member 12Aas well as a lateral force. (It will be appreciated that the “V” shapedelastic member could be replaced with two separate elastic pieces.) Afastening block 34 may be secured by adhesive or other fastener to theinner wall of the tubular frame member 130, 132, 134 and 136 to providea surface for securing the mid-point 36 by a bolt or other fastener. Inthe illustrated embodiment, fastening assemblies 40 are located in pairson opposite sides of the inner frame section 12A such that substantiallyequal but opposite forces are applied to the inner frame section 12A bythe elastic suspension members 32 so that the inner frame section 12Anormal resting position is in the center of the coil passage 124 definedby tubular frame member 130, 132, 134 and 136. In one exampleembodiment, the elastic suspension members 32 in a split tubular framemember 130, 132, 134 and 136 are all secured to one half thereof (forexample a bottom half) to facilitate securing the inner frame section12A in place before closing up the receiver coil passage 124 with theother half.

Elastic members 32 can be formed from rubber or other suitable elasticor resilient material. The fastening assembly 40 could take manydifferent configurations than is shown in FIGS. 6 and 7 to elasticallysuspend the inner frame member 12A.

In some embodiments the inner frame member 12A has a V-shapedcross-section and defines an open-sided trough 42 that provides an innercable passage 44 in which the receiver coil 16A is received. In someexample embodiments, the inner frame member 12A could alternatively havea semi-rectangular, or semi-circular or circular or othercross-sectional area. In at least some embodiments the receiver coil 116is a loop or multi-turn coil formed that is secured in the trough 42 bytape and/or other type of fastening mechanism.

Referring to FIGS. 8 and 9, in another alternative embodiment, a doublesuspension system is used to suspend the receiver coil interior frame12A within the outer tubular frame members 130, 132, 134 and 136 ofreceiver coil assembly 112. Although shown as having a cylindrical crosssection, tubular frame members 130, 132, 134 and 136 can have othercross-section configurations as shown in previous Figures. In the doublesuspension configuration of FIGS. 8 and 9, the suspension assemblies 40located along the lengths of each of the internal frame members 12A areconnected to intermediate frame members 74, which are in turn suspendedfrom the outer frame members 130, 132, 134 and 136. For example, firstelastic suspension members 32 opposingly suspend the inner frame member12A in the center of a cylindrical or semi-cylindrical intermediateframe section 74, which is then centrally suspended in a similar mannerby further elastic members 76 that extend between the intermediate framesection 74 and the outer frame member 130, 132, 134 or 136. As can beseen in FIG. 9, the further elastic members 76 can also be arranged inV-shaped pattern to act against longitudinal movement as well as radialmovement in a similar manner as the first elastic suspension members 32.As noted above, intermediate frame section 74 can be semi-cylindrical,which allows easy access to the interior of the frame section 74 duringassembly and repair.

Thus, in the embodiment of FIGS. 8 and 9, the inner frame section 20Athat supports receiver coil 116 is suspended by a number of firstelastic suspension members 32 to a number of respective intermediateframe sections 74 which are in turn suspended by one or more secondelastic suspension members 76 (which may for example be formed fromrubber) from the outer frame 14A. The inner frame section 12A mayfurther be positioned at or near the centre of the passageway 124.Regions that are (i) proximate the connections between the firstsuspension members 32 and each of the inner frame section 12A and theintermediate frame sections 74, and (ii) proximate the connectionsbetween the second suspension members 76 and each of the intermediateframe sections 74 and the outer frame members can be coated with afriction reducing agent such as silicone. A silicone coating may reducethe noise caused by rubbing at the attachment or connection point. Insome example embodiments, the first suspension members may be connectedto the respective frame sections by cable ties that pass throughpre-drilled holes or attached loops. Alternatively, any number of otherpossible methods can be used to attach the first and second suspensionmembers including: hooks, or a machined hook-like attachment pointconnected to the attachment points whereby the suspension members may belooped around the hooks and then covered by silicone; alternatively,loops on the first and second suspension members can be screwed into theattachment points; another possibility is to glue the first and secondsuspension members to the inner frame, and to the outer frame orintermediate frame sections.

As shown in FIGS. 8 and 9 both the first and second suspension members32 and 76 extend at an angle other than 90 degrees to both radially andlongitudinally bias the inner frame section 12A and the receiver coils116 in a central position in internal receiver coil passages 124,respectively. The single and double suspension arrangements that arediscussed above may in at least some embodiments improve the signal tonoise ratio SNR of the receiver coil assembly by reducing the effects ofvibration of the receiver coil assembly on the receiver coil. In otherexample embodiments, other support mechanisms can be used includingtriple-suspension, springs, surrounding the coil with foam, or othermeans of positioning the coil in the centre of the inner frame in amanner that reduces noise

FIG. 10 illustrates another possible suspension configuration for theinternal frame 12A. The configuration shown in FIG. 10 is similar tothat descried above in respect of FIGS. 6 and 7, with the addition ofintermediate rigid rods 94 that run longitudinally in passage 124between the opposite sides of the internal frame 12A and the walldefined by the outer frame member 130, 132, 134 or 136. The elasticsuspension members 32 on one side of the internal frame 12A are eachattached at approximately a midpoint 98 to one of the rigid rods 94, andthe elastic suspension members 32 on the opposite side of the internalframe 12A are each attached at a midpoint 98 to the other of the rigidrods 94. The rods 94 can tie the suspension members together to spreadforces applied to any individual member among a number of suspensionmembers. Additionally, the rods 94 themselves can act as energyabsorbing structures. The rods 94 can in some embodiments be broken intosmaller rod sections.

Further example embodiments of coil suspension systems suitable for usein the receiver coil assembly 112 can be seen for example in patentapplications PCT/CA2009/000607 and U.S. Ser. No. 12/118,194, therespective contents of which are incorporated herein by reference.

Using a rectangular frame with open center configuration such as thereceiver coil tow assembly 112 can, in at least some configurations,support a relatively large loop receiver coil 116 in a light weight andaerodynamic manner when compared for example to torpedo-style birds thathave in the past been towed from fixed-wing aircraft.

Although the receiver coil tow assembly has been described as having arectangular loop configuration with a central support member, otheropen-centered frame styles may alternatively be used, including forexample triangular shaped configurations, five or more sided simplepolygonal shaped configurations, or circular or oval or ellipticalshaped configurations, among others.

In some example embodiments, the geophysical electromagnetic (EM) surveytow assembly system 100 is configured for use in an AFMAG-typegeophysical prospecting system that depend on tipper or tilt anglemeasurements as shown for example in above-mentioned U.S. Pat. No.6,876,202, incorporated herein by reference. In such an application,attitude sensors can be located on the receiver coil assembly 112 sothat the orientation of such assembly can be detected and theorientation information used in the calculation of tilt angleinformation that is derived from the signals collected from the receivercoil assembly. For example, one or more accelerometers can be secured tothe coil assembly 112 to determine attitude information. Alternatively,GPS receivers can be placed at spaced apart locations on the receivercoil assembly in order to track its attitude.

In this regard, FIG. 5 illustrates an AFMAG survey system 200 accordingto an example embodiment that incorporates the geophysicalelectromagnetic (EM) survey tow assembly system 100. As noted above,AFMAG systems measure EM fields resulting from naturally occurringprimary signal sources. The AFMAG system 200 includes geophysicalelectromagnetic (EM) survey tow assembly system 100 and a groundassembly 14. The geophysical electromagnetic (EM) survey tow assemblysystem 100 is mounted to a fixed wing aircraft to be towed over a surveyarea and includes receiver coil 116 and a low noise amplifier 18. In anexample embodiment the receiver coil 116 is configured to have avertical dipole orientation during flight in order to provideelectromagnetic field measurements in the Z axis. The tow assemblysystem 100 is connected to signal processing equipment that is generallydisposed inside the aircraft such as a computer 22 that includes ananalog to digital converter device (ADC) 24 connected to receive theoutput of the low noise amplifier 18. The on-aircraft computer 22 isequipped with one or more storage elements that can include RAM, flashmemory, a hard drive, or other types of electronic storage, and may beconfigured to perform data processing functions on signals received fromsensor 16.

In an example embodiment, the tow assembly system 100 also includes aspatial attitude detection device 28 to compensate for the roll, pitchor yaw of air assembly 12 and particularly coil 116 in flight that cancause anomalies in measurement of the tilt angles produced by theelectromagnetic fields by electromagnetic sensor coil 116. The spatialattitude detection device 28 includes inclinometer devices for measuringthe roll, pitch and yaw of the coil assembly 112 and particularly sensorcoil 116 during flight at any given moment. In addition for yawmeasurements, the spatial attitude detection device 28 may comprise adevice for tracking the flight path such as a compass utilizing thedirection of the geomagnetic field vector. In example embodiments, thecoil assembly 112 or host aircraft 104 can include a Global PositioningSystem (“GPS”) device such that data obtained from sensor coil 116 andspatial attitude detection device 28 can be correlated with geographicalposition and GPS time and ultimately used either at computer 22 or aremote data processing computer 26 to correct the measurements of theelectromagnetic field tilt angles to reflect the movements of the coilassembly 112 and particularly sensor coil 116, and correlate theelectromagnetic field data obtained from sensor 116 with the spatialattitude data of coil assembly 112. This allows the creation of surveydata that can be adjusted based on variations of the spatial attitude ofthe sensor coil 116 during flight.

In an example embodiment, the airborne equipment also includes ageographic relief measurement device 36 connected to the airbornecomputer 22 in order to allow compensation for geographical relief thatcould otherwise distort horizontal magnetic fields by producing falseanomalies of tilt angles even where there are very homogeneous rocksbeneath the ground surface. Geographic relief measurement device 36collects data for post flight (or in some cases real-time) calculationsof the tilt angles of geographical relief in the survey area. In oneexample embodiment, the geographic relief measurement device 36 includesa first altimeter device that provides data regarding absolute altitudeof the airborne sensor 16 above a fixed reference (for example sealevel) and a second altimeter device that providing data regarding therelative altitude of the of the airborne sensor 16 above the actualsurvey terrain. Comparing the relative altitude data and absolutealtitude data in the local co-ordinate system of the survey area allowsan evaluation of the geographic relief of the survey area that can beused to calculate the tilt angles of the survey area geographic relief.

The ground assembly 14 is configured to be placed on a stationary basepoint, and includes at least a pair of electromagnetic sensors 17connected through a low noise amplifier 19 to a ground assembly computer23. In an example embodiment the electromagnetic sensors 17 are receivercoils configured to provide electromagnetic field measurements in the Xand Y axes. The computer 23 includes an analog to digital converterdevice (ADC) 25 connected to receive the output of the low noiseamplifier 19, and is equipped with one or more storage elements that caninclude RAM, flash memory, a hard drive, or other types of electronicstorage, and may be configured to perform data processing functions onsignals received from sensors 17. The ground assembly can also include aGPS receiver so that the X and Y axis data received from sensors 17 canbe time stamped with a GPS clock time for correlation with the Z axisdata that is recorded by airborne computer 22. (Z-axis being thevertical axis and X and Y being orthogonal horizontal axis.)

In an example embodiment, the data collected by airborne computer 22 andthe data collected by the ground computer 23 is ultimately transferredover respective communication links 30, 32 (which may be wired orwireless links or may include physical transfer of a memory medium) to adata processing computer 26 at which the electromagnetic field dataobtained from sensors 16 and 17, the attitude data from spatial attitudedetection device 28, data from geographic relief measurement device 36,and the GPS data from GPS sensors associated with each of the airassembly 12 and ground assembly 14 can all be processed to determine thetipper attributes for the survey sight using techniques as set out forexample in U.S. Pat. No. 6,876,202. Such information can them be used todetermine conductivity patterns for the survey site to identifyanomalies for future exploration.

Accordingly, in one example embodiment the receiver coil assembly 112(also referred to as the “bird”) will now be described by way of nonlimiting example. In such example, the bird 112 houses multi-turnrectangular loop or coil 116 together with its suspension system andelectronics. In towed flight the axis of the coil 116 is nominallyvertical. The size of the loop is limited by the space available beneaththe towing aircraft for stowing the loop during takeoff and landing. Inthe case of a Cessna 208B towing aircraft, loop dimensions may by way ofnon-limiting example be 3 m center-to-center in the lateral directionand 4 m center-to-center in the longitudinal direction. In the case of alarger towing aircraft, a larger loop, for example up to 8 m laterallyand longitudinally, may be used to provide improved signal to noiseratio. Smaller loops, for example 3m laterally and longitudinally, mayprovide useful results when used with smaller aircraft. The central openarea 122 has dimensions just less than that of the coil 116. The mass ofthe loop, suspension system, and electronics may for example beapproximately 60 kg, distributed approximately uniformly around thereceiver coil support frame, while in some embodiments the bird may havea total mass of 150 kg, more or less. In one example, the bird 112contains an enclosed clear passageway 224 with at least a circular crosssection 0.22 m inside diameter for installation of the loop. Multipleattachment points are provided on the inner surface of the passageway224 for suspending the coil 116. The bird 112 can be split along ahorizontal plane to open the passageway to allow the loop 116 andsuspension to be installed or serviced without breaking the loop.

In some example embodiments, the bird 112 is constructed of materialsthat are electrically insulating and non-magnetic, except that fastenersmade of brass, aluminum or 316 stainless steel may be used. The bird 112components are streamlined and faired to minimize vibration caused byairflow past the bird 112. In one example of a survey flight, the bird112 is towed from a fixed aircraft such as, for example, a Cessna 208BCaravan aircraft at 80-120 knots airspeed on 100 m of cable. Fortakeoff, cruise, and landing the bird 112 is secured in a cradle (latchsystem 150) attached to the bottom of the aircraft fuselage. In someexample embodiments, the tow cable 110 can be an electromechanical cableincluding a load bearing cable with a conductor equivalent to RG58A/Ucoaxial cable or better, or alternatively it may contain at least fourtwisted pairs of at least AWG 20. The load bearing cable of tow cable110 is made substantially of non-magnetic materials.

The tow and latch assembly includes a hydraulically powered winch system105 installed on the aircraft 104 which deploys the bird 112 andrecovers it to the cradle while in flight under control of an operatorin the aircraft. In one example, in towed flight, the bird 112 isaerodynamically stable and the pitch and roll attitude of the passageway224 for the loop 116 is horizontal±5°. In some example embodiments, thetow point on the bird can be manually changed fore and aft while on theground by adjusting the position of tow cable fastener 144 to achieve ahorizontal flight attitude. An alternative to the electromechanical towcable is to use a non-electrical tow cable and to use a battery operatedradio telemetry system to transmit data to the aircraft.

It will be appreciated by those skilled in the art that other variationsof the embodiments described herein may also be practiced withoutdeparting from the scope of the invention. Other modifications aretherefore possible.

1. An airborne geophysical electromagnetic (EM) survey tow assemblysystem for use with an aircraft, comprising: a substantially rigidreceiver coil frame that defines a central open area, the receiver coilframe forming a continuous internal passageway that extends around thecentral open area; a receiver coil housed within the internal passagewayisolated from the central open area, the receiver coil being configuredto measure the response of surveyed terrain to naturally occurring EMevents; and a tow cable for suspending the receiver coil frame from theaircraft, the receiver coil frame being formed from rigid membersconfigured to provide aerodynamic stabilization to maintain the receivercoil frame in a desired pitch and yaw orientation relative to adirection of travel and the horizontal when the receiver coil frame issuspended during flight.
 2. The system of claim 1 wherein the rigidmembers are configured to provide aerodynamic stabilization to maintainthe internal passageway of the receiver coil frame with a pitch and rollattitude of horizontal±5° when the receiver coil frame is suspendedduring flight.
 3. The system of claim 1 wherein the rigid members of thereceiver coil frame comprise a hollow frame member with a wing-likeconfiguration, through which a portion of the internal passagewayextends.
 4. The system of claim 3 wherein the rigid members of thereceiver coil frame comprise at least one vertically extendingstabilizing fin member.
 5. The system of claim 3 wherein the hollowframe member with the wing-like configuration is located proximate atrailing end of the receiver coil frame and vertical stabilizing finsextend from the hollow frame member.
 6. The system of claim 5 whereinthe receiver coil frame is a polygonal structure including a pair ofspaced apart tubular side frame members interconnected at a front endthereof by a front tubular frame member and interconnected at a back endthereof by the hollow frame member with the wing-like configuration. 7.The system of claim 6 wherein the receiver coil frame is substantiallyrectangular.
 8. The system of claim 6 wherein the rigid members comprisea central support member that extends across the central open areabetween the front tubular frame member and the hollow frame member, thetow cable being attached to the central support member.
 9. The system ofclaim 8 wherein the only location of the receiver coli frame that thetow cable is attached to is the central support member.
 10. The systemof claim 1 further comprising a cradle for securing to an underside of afixed wing aircraft and adapted to engage the receiver coil frame, and awinch system connected to the tow cable and configured to extend the towcable to suspend the receiver coil frame from the fixed wing aircraftduring a survey and to retract the tow cable to draw the receiver coilframe into a retracted position in engagement with the cradle at theunderside of the fixed wing aircraft during takeoff and landing.
 11. Thesystem of claim 10 wherein the cradle includes one or more releasablelatch members for releasably latching the receiver coil frame when thereceiver coil frame is drawn into the retracted position.
 12. The systemof claim 1 comprising signal processing equipment for receiving signalsfrom the receiver coil that are representative of EM fields generated inresponse to naturally occurring electrical events.
 13. An airbornegeophysical electromagnetic (EM) survey system, comprising: an aircraft;a substantially rigid receiver coil frame that defines a central openarea, the receiver coil frame forming a continuous internal passagewaythat extends around the central open area; a receiver coil housed withinthe internal passageway isolated from the central open area; a tow cablefor suspending the receiver coil frame from the aircraft; and signalprocessing equipment in communication with the receiver coil to receivesignals therefrom representative of EM fields generated by a surveyedterrain in response to naturally occurring electrical events; thereceiver coil frame being formed from rigid members configured toprovide aerodynamic stabilization to maintain the receiver coil frame ina desired pitch and yaw orientation relative to a direction of traveland the horizontal when the receiver coil frame is suspended duringflight.
 14. The system of claim 13 wherein the rigid members areconfigured to provide aerodynamic stabilization to maintain the internalpassageway of the receiver coil frame with a pitch and roll attitude ofhorizontal±5° when the receiver coil frame is suspended during flight.15. The system of claim 13 wherein the rigid members of the receivercoil frame comprise a hollow frame member with a wing-likeconfiguration, through which a portion of the internal passagewayextends.
 16. The system of claim 15 wherein the hollow frame member withthe wing-like configuration is located proximate a trailing end of thereceiver coil frame and vertical stabilizing fins extend from the hollowframe member.
 17. The system of claim 16 wherein the receiver coil frameincludes a pair of spaced apart tubular side frame membersinterconnected at a front end thereof by a front tubular frame memberand interconnected at a back end thereof by the hollow frame member withthe wing-like configuration.
 18. The system of claim 13 wherein theaircraft is a fixed wing aircraft and the system comprises: a cradle forsecuring to an underside of a fixed wing aircraft and adapted to engagethe receiver coil frame; and a winch system secured to the aircraft andthe tow cable and configured to extend the tow cable to suspend thereceiver coil frame from the fixed wing aircraft during a survey and toretract the tow cable to draw the receiver coil frame into a retractedposition in engagement with the cradle at the underside of the fixedwing aircraft during takeoff and landing.